I went for a 5k TT yesterday(on a 400m track), it was a failure. Plan was to start at 4.05/km and go 5s faster each km till I would reach 3.55/k and mantain for the last 2k. Well, I had to stop after 3k because I was running out of breath... Temp was 15 deg C, so perfect.

Did another TT today on the same 400m track, it was much better, time was 20:10 after 12.5 laps. The watch recorded 5.1km... Calves feel pretty tight now.

Anyway, I wanted to ask you about HR zones.
My current training zones are based on % of HR max which is 191bpm. I had the max of 191 a few months ago, but only once. For example today it was 189bpm. So for the easy runs I try to stay in aerobic Z3(133-153bpm) to keep my HR at around 149bpm(+/-3bpm).

My watch also estimates LTHR, which is currently estimated at 4.27/km @ 171bpm. If I use LTHR to calculate my training zones, then the aerobic Z3 zone is between 152-162bpm.

So, should I switch to LTHR zones calculation method(which will increase the pace for the easy runs), or stay with %HRmax? Which method is better?

I ran this session on a 400m track and lapped my watch manually after each 400m lap. Target was 1.42/lap for the first 2k; for the rest I tried 1.40/lap.
It felt harder than the previous 2k sessions. I think it's because my watch doesn't measure distances properly. For this reason I decided I do all my quality sessions on the 400m track.

Ed, again, thanks for the info you provide ;-).
Regarding 'not all runners are the same', I don't really fit in your 'rule of thimb' regarding easy runs.
For example, if I go on an easy run at a pace 1min slower than my 10pace, say 5.10-5.20/km my HR will definetly go up to 160. I will do a test next week to see what's my avg hr at a 5.10/km pace.
At the moment, easy run paces are around 5.40-5.50/km with avg hr being around 149bpm(upper Z3 aerobic zone).

I entered a race this Saturday on 21'th October, 22km trail, 700m elevation. So I figured I'd do the last week of the cycle, but instead the 10x400m today, I did 3x5min @ race pace with 2.30min easy in between. On Thuesday I'll do the 6x1min session.

A bit busy with work as we have to move to a new building. Gotta do some overtime...

Anyway, race was great, I managed to beat my last year's time by 9min with a time of 1h:59m:50s, I have just barely finished in the sub 2h target. The problem was that I tripped and fell... No injuries, but I lost a few places. I finished on 50'th place out of 343 runners. Lots of good runners entered.

I'm thinking of putting aside the cycles for next months as I've been on them since March I believe. Any thoughts?